An integrated study (inoceramid bivalves, ammonites, calcareous nannofossils, planktonic foraminifera,
stable carbon isotopes) of the Ten Mile Creek section, Lancaster, Dallas County, north Texas, a candidate Global boundary Stratotype Section
and Point for the base of the Santonian Stage
ANDREW S. GALE1, WILLIAM J. KENNEDY2, JACKIE A. LEES3, MARIA ROSE PETRIZZO4& IRENEUSZ WALASZCZYK5
1School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK. E-mail: asg@nhm.ac.uk
2Oxford University Museum of Natural History, Parks Road, Oxford OX1 3PW, UK.
E-mail: Jim.Kennedy@oum.ox.ac.uk
3Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK
4Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy
5Institute of Geology, University of Warsaw, Al. ˚wirki i Wigury 93, PL-02-089 Warsaw, Poland
ABSTRACT:
GALE, A.S., KENNEDY,W.J., LEES, J.A., PETRIZZO, M.R. & WALASZCZYK, I. 2007. An integrated study (inoceramid bivalves, ammonites, calcareous nannofossils, planktonic foraminifera, stable oxygen and carbon isotopes) of the Ten Mile Creek section, Lancaster, Dallas County, north Texas, a candidate Global boundary Stratotype Section and Point for the base of the Santonian Stage. Acta Geologica Polonica, 57 (2), 113-160. Warszawa.
The WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas, exposes a 23 metre section of Austin Chalk that can be integrated into a more than 60 m composite sequence for the Dallas area on the basis of bed-by-bed correlation. The section was proposed as a possible candidate Global Boundary Stratotype at the 1995 Brussels meeting on Cretaceous Stage boundaries, with the first occurrence of the inoceramid bivalve Cladoceramus undulatoplicatus (ROEMER, 1852) as the potential boundary marker. An integrated study of the inoceramid bivalves, ammonites, planktonic foraminifera, and calcareous nannofos- sils places the first occurrence of Cl. undulatoplicatus in a matrix of ten ancillary biostratigraphic markers. The candidate Global boundary Stratotype Section and Point (GSSP) is located within a composite stable carbon isotope curve for the Austin Chalk as a whole. This shows it to lie 3.5 m below the Michel Dean stable car- bon isotope event, originally recognised in the English Chalk. The first occurrence of Cl. undulatoplicatus lies in the same position in relation to stable carbon isotope events in both Texas and England that can in prin- ciple be recognised globally in marine sediments. The WalMart section satisfies many of the criteria required of a GSSP for the base of the Santonian Stage, although ownership and access require clarification.
Key words: Ammonites, Inoceramid Bivalves, Foraminifera, Calcareous Nanno- fossils, Carbon Isotopes, Santonian, Texas.
INTRODUCTION
At the meeting of the Santonian Working Party, held at the Second International Symposium on Cretaceous Stage Boundaries in Brussels, 8-16th September, 1995, the following statements were made with respect to the base of the Santonian Stage:
‘Primary marker: The lowest occurrence of Cladoceramus undulatolplicatus. It is a taxon easily recognisable and widespread. It is known from N.
America, Europe, Africa, Madagascar, and central Asia.
This proposal was supported by a majority at Brussels. (Yes=23; no=1; Abstentions=2).
Postal vote; 20 votes (out of 39 WG members) were returned. (Yes =17; No =1; Abstentions=2).’
‘Boundary Stratotype Section: As yet, we can- not make a formal proposal, because we need to know and integrate the biostratigraphy better. We have selected three candidates:
1) Olazagutia Quarry (Navarra, Spain). M.
LAMOLDAwould collate data and report to the Chairman.
2) Seaford Head (Sussex, England). R. MORTI-
MORE and C. J. WOOD would collate data and report to the Chairman.
3) Ten Mile Creek (Dallas, Texas). E.G. KAUFFMAN
and A.S. GALEwould collate data, and report to the Chairman.
This proposal was supported UNANIMOUSLY.
(yes=34; No=0; Abstentions=1).
The postal ballot supported this proposal (Yes=18; No=1; Abstention=1) (LAMOLDA &
HANCOCK1996, pp. 99-100).’
Twelve years on, the purpose of the present contribution is a simple one: to document the candidate boundary section in Ten Mile Creek, Lancaster, Dallas County, Texas, and place the first occurrence of Cladoceramus undulatoplica- tus (ROEMER, 1852) (Text-fig. 1), which was originally described from the Austin Chalk of New Braunfels, Texas, in its stratigraphic con- text.
Throughout this account we use first occur- rence (FO) and last occurrence (LO) to describe the limits of the range of taxa based on our present observations, unless indicated otherwise.
Fig. 1. The lectotype of Cladoceramus undulatoplicatus (ROEMER, 1852), PIB ROEMERCollection 03, the original of ROEMER(1852, pl. 7, fig. 1), from the Austin Chalk on the banks of the Guadalupe River below New Braunfels, Comal County, Texas. Geologisches
und Paläontologisches Intitut Collections, Universität Bonn
THE AUSTIN CHALK IN DALLAS COUNTY, TEXAS
The Austin Chalk Formation is approximately 161 metres (530 feet) thick in Dallas County, Texas (Text-fig. 2), and is a rhythmically bedded, bioturbated hemipelagic chalk with a variable clay content of about 5-30%. It rests with marked dis- conformity upon the Eagle Ford Shale Formation.
The Austin Chalk has been divided into three members, successively, the Atco Chalk, the Bruceville Marly Chalk and the Hutchins Chalk (LARSON & al. 1991). The Atco Chalk is rich in carbonate, and contains a number of thin, hard limestones and several bentonitic clays, represent- ing altered volcanic ashfalls. The Bruceville Marly Chalk has a higher clay content, and therefore does not form such good exposures. The Hutchins Chalk was not investigated in Dallas County dur- ing the present study, but is evidently similar to the carbonate-rich hemipelagic chalk facies devel- oped 100 km to the south, at Waxahachie in Ellis County (GALE& al. in press).
The Atco and lower Bruceville members dis- play rhythmic alternations of more and less marly
chalk on a decimetre to metre scale, which are very conspicuous in weathered outcrops. These are rather irregular in thickness, and were interpreted by LARSON & al. (1991) as representing climatic cyclicity in the Milankovitch Band, with frequen- cies corresponding to the mode of precession at 20 kyr. LARSON& al. also tentatively identified long and short eccentricity signals (400 and 100 kyr) in the Atco. The upper part of the Atco Member also contains several broad, shallow erosional channels, about 5-10 m across and 1-1.5 m deep, which dis- play bedded chalk infills.
The Austin Chalk ranges from Late Turonian to earliest Campanian in age (HANCOCK &
WALASZCZYK 2004; GALE & al. in press). The Atco-Bruceville boundary falls well within the Late Coniacian, not at the Coniacian-Santonian bound- ary as reported by LARSON & al. (1991). Their assignation was based upon presumed occurrences of Santonian inoceramids, expected to co-occur with the ammonite Texanites sp. and has not been supported in the present study. The succession described herein spans the boundary of the Atco and Bruceville members and is of Late Coniacian to Early Santonian age.
Fig. 2. Locality map of the Dallas area, Texas, showing the location of sections described in the text
LOCALITY DETAILS Church of the Nazarene
This section comprises cuttings in the sides and floor of Ten Mile Creek for approximately 200 m north of the intersection of the Creek with Pleasant Run Road in Desoto, Dallas County. Access is pos- sible behind the Church of the Nazarene, on the east side of the Creek (Text-fig. 3). Access to the lower section is only possible when the water level is low, and requires some shallow wading. The sec- tion was numbered 9 by WILLIAMS (1957) and referred to as the Williams 9 section by LARSON&
al. (1991). The precise relationships of the base of this section with the underlying part of the Atco Member are uncertain, but LARSON & al. (1991) argued that a small gap of a few metres separates the base of this section from the underlying Jacobsen 3 locality to the north. The northern part of the Nazarene section is bisected by an approxi-
mately east-west fault, which downthrows the Bruceville Marly Chalk Member against the upper part of the Atco Member. From detailed strati- graphical correlation, the fault can be calculated to downthrow approximately 28 m to the north.
The (stratigraphically) lower part of the section exposes 15 m of uppermost Atco Member, and 2 m of basal Bruceville Member (Nazarene 1: Text-fig.
4). The highest part of the succession is exposed in bluffs adjacent to the Church of the Nazarene, and progressively lower levels are seen in the floor and sides of the creek to the north. The lower 7 m com- prise rhythmic, strongly bioturbated alternations of paler grey chalks and darker grey marls of variable thickness. The upper part (7-15 m) is made up of regular alternations of hard limestones and thin, dark, recessing marls. A conspicuous channel, about 1 m in depth, is present between 8 and 9 m (LARSON& al. 1991, fig. 29a, b). A supposedly ben- tonitic yellowish marly clay is present approximate- ly 0.3 m beneath the summit of the Atco Member.
Fig. 3. The location of the Church of the Nazarene and the WalMart candidate GSSP section on Ten Mile Creek, Dallas County, Texas
Fig. 4. The composite section of the upper Atco and Bruceville members of the Austin Chalk, based on sections on Ten Mile Creek, and in Arbor Park, Dallas County, Texas. Cl:
Cladoceramus undulatoplicatus; M. a.: Menabites (Australiella) austinensis; P.p.: Protexanites (Protexanites) planatus T.g.: Texanites (Texanites) gallicus; T.v.: Texanites (Texanites) vanhoepeni.
Scales are in metres
The uppermost 2 metres of the succession are more marly and are assigned to the basal Bruceville Marly Chalk Member. The uppermost 5 m of the Nazarene section overlaps with the base of the WalMart locality to the south-east (see below) and all the beds in this interval can be con- fidently correlated between the two localities.
To the north of the fault, 14 m of Bruceville Member are exposed (Nazarene 2: Text-fig. 4). The lower part of the succession (0-3.5 m) comprises inconspicuously rhythmic, weakly indurated grey marls. These are overlain by four alternations of paler limestones and darker marls (3.5-6 m), then a succession of marls and weakly developed lime- stones (6-12.3 m). The section is capped (at 12.3-14 m) by a group of 3 marly limestones containing abundant specimens of the inoceramid Clado- ceramus undulatoplicatus (ROEMER, 1852). The lower 6 m of this section overlap with the upper- most part of the WalMart section, and individual
beds can be matched precisely between the two localities. The uppermost 6 m of the Nazarene 2 Bruceville succession also can be correlated bed by bed to the Arbor Park section in north Dallas (see below).
WalMart
This section (Text-figs 3, 4, 6) comprises low bluffs in the sides of Ten Mile Creek, 1-2 km east- south-east of the WalMart store north of Main Street, Lancaster, Dallas County, east of the free- way 35E.The section has only been briefly men- tioned in the literature as localities 40/48 of JACOBSEN(1961). Access to the Creek is most eas- ily obtained from the south side of Main Street opposite the WalMart store, and the section can be easily worked when the water level is low.
The lower section comprises a series of bluffs on the northern side of the creek, and exposes the
Fig. 5. Locality map for the Arbor Park sections in Dallas County, Texas
Fig. 6. The WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. The ranges of key marker macrofossils are indicated, as is the position of the candidate GSSP for the base of the Santonian Stage, the first occurrence of Cladoceramus undulatoplicatus (ROEMER, 1852).
Scale is in metres
uppermost 3 m of the Atco Member and approxi- mately the lowest 7 m of the Bruceville Member, dipping gently to the south-east. The beds making up the lower 5 m of the section, including the puta- tive bentonite (2.8 m), exactly match those exposed adjacent to the Church of the Nazarene. At 8.2 m, a conspicuous 0.3-0.35 silty limestone containing oys- ters is present. The succession continues in low cliffs to the south-east on the southern side of the Creek, and an erosion channel is present at 11 m. The over- lying strata (11-21 m) comprise poorly defined beds of marl and marly limestone, containing oysters and inoceramid bivalves at some levels. The candidate GSSP falls at the 18.4 m level in this succession, and is marked by the first occurrence of Cladoceramus undulatoplicatus. The uppermost 3 m exposed include four thin limestones, which alternate with dark grey marls containing pyrite nodules, and cor- relate with beds exposed to the north of the fault in the Nazarene section (see above).
Arbor Park
The uppermost part of the succession described here is exposed in the sides of the un-named creek that runs through Arbor Park in north Dallas (Text-figs 4, 5). A total of 11 m of Bruceville Member are exposed here. The lower, marly part of the succession correlates well with the Nazarene 2 succession in Desoto, and correlation is con- firmed by the distinctive triplet of marly limestones full of Cladoceramus undulatoplicatus present between 4.7 and 6.2 m at Arbor Park and between 12.3 and 14 m in the Nazarene section. The higher part of the Arbor Park succession is made up of rhythmic alternations of marls and marly lime- stones, and a possibly bentonitic clay is present at 9.2 m.
THE COMPOSITE DALLAS SECTION
The detailed bed-by-bed logs of the Nazarene, WalMart and Arbor Park sections can be precisely correlated on the basis of marker beds, as dis- cussed in the previous section. These provide the basis for the composite log shown in Text-fig. 4, which places the WalMart candidate GSSP section in its regional stratigraphic context of approxi- mately 50 m of the Atco and Bruceville members of the Austin Chalk Formation.
REPOSITORIES OF MATERIAL STUDIED GPIM: Geologisch-Paläontologisches Institut und Museum, Münster;
GWB: Geologische Abteilung der Westfälischen Bergewerkschaftenkasse, Bochum;
NLBH: Niedersächsiches Landesamt für Bodenforschung, Hannover
OUM: Oxford University Museum of Natural History;
PIB: Paläontologisches Institut und Museum of the Friedrich Wilhelms University, Bonn;
TMM: Texas Memorial Museum, Austin, Texas.
UWFG: Faculty of Geology, University of Warsaw, Warsaw.
Foraminiferal samples are housed in the collec- tions of the Sezione di Geologica et Paleontologia of the Dipartimento di Scienze della Terra “Ardito Desio”, Milan.
Nannofossil samples are housed in the Micropalaeontology Unit, Department of Earth Sciences, University College, London.
Stable isotope samples are housed in the Geological Collections, Oxford University Museum of Natural History.
INOCERAMID BIVALVES (I. WALASZCZYK) Introduction
The WalMart section on Ten Mile Creek offers an exceptionally rich inoceramid record across the Coniacian - Santonian boundary interval. It pro- vides a representative record for the southern Euramerican biogeographical region (biogeo- graphic terminology used here is that defined by KAUFFMAN 1973), characterized by a Magadi- ceramus-dominated Upper Coniacian and a Cladoceramus-rich Lower Santonian, south of the regular occurrence of Sphenoceramus. The change to the Cladoceramus fauna, marking the base of the Santonian, is quite rapid, but the inoceramid suc- cession appears to be complete in terms of that cur- rently recognised elsewhere in this interval.
Most of the material from the WalMart section comes from four fossiliferous horizons (levels around 11 m, 17 m, 18.4-18.5 m and 22 m) but the fauna is in no way limited to them. Inoceramids are quite well preserved, although specimens from marly beds are usually crushed.
The late Coniacian inoceramid assemblages are dominated by Magadiceramus, represented mostly by M. crenelatus (SEITZ, 1970) (Text-figs 7.1-7.2, 7.6-7.7, 7.9; 8.3-8.4) and M. complicatus (HEINE, 1929) (Text-figs 7.3, 7.4, 8.6). Magadiceramus sub- quadratus (SCHLÜTER, 1887) (Text-figs 7.5, 7.8) is rare. There are also two specimens of M. cf. crenis- triatus (Text-figs 8.1, 8.5), a characteristic albeit poorly recognised magadiceramid species, from the 11.3 m level, and from the 18 m level, just below the candidate GSSP, the FO of Cladoceramus undulatoplicatus in the section, at the 18.4 m level. A number of juvenile magadice- ramid specimens (mostly fragments) are specifical- ly indeterminate and left in open nomenclature as Magadiceramus sp. Non-magadiceramid inoce- ramids are rare; there are only two specimens from a level just below the candidate GSSP in the mate- rial at hand. These are Inoceramus sp. (Text-fig.
9.8), which is compared here to Inoceramus anom- alus (HEINE, 1929), and a platyceramid species (Text-fig. 9.7), which clearly belongs to the mantel- li group. The former is rarely recorded, but it is worthy of note that single specimens of this species, from an equivalent stratigraphical level, have recently been described from the Pueblo sec- tion in Colorado in the US Western Interior (WALASZCZYK & COBBAN 2006, 2007). The exact stratigraphic horizon of the German type specimen is unknown.
Cladoceramus undulatoplicatus (Text-figs 1, 9.4 and 10.2), the boundary marker for the base of the Santonian, first appears at the 18.4 m level, and is quite common in the overlying metre of section. It becomes less common in the higher parts of the WalMart succession. A single specimen, referred to herein as Cl. cf. undulatoplicatus comes from slightly above the 19 m level. There is a second abundance peak in the Bruceville Member in other sections: between 12.5 and 14 m in the Nazarene 2 section, and between 4.6 and 8 m in the Arbor Park section. The lowest Cordiceramus comes from the 18.4 m level; these are juvenile fragments, referred here to C. arnoldi (SEITZ, 1961), a member of the bueltenensis group.
Cordiceramids dominate the highest assemblage studied, from the 22 m level (Text-figs 9.1-9.3, 9.5, 9.6, 9.9-9.10). This also marks the first occurrence in the section of the characteristic Santonian inoceramid Platyceramus cycloides (WEGNER, 1905) (Text-fig. 10.1).
Correlation
Correlation of the WalMart sequence with other sections in the southern part of the Euramerican biogeographical region is straightfor- ward. Problems may, however, arise when attempt- ing precise correlation with sections in the north- ern part of the region, because of the absence or irregular occurrence of Cl. undulatoplicatus.
Sphenoceramids of the cardissoides-pachti group, the approximate markers for the Coniacian- Santonian boundary interval in this area, are known to make their first appearance slightly below the first occurrence of cladoceramids (SEITZ 1965). Other potential inoceramid markers are Cordiceramus of the cordiformis group, and repre- sentatives of the Platyceramus cycloides group, which appear at approximately the same level as the first Cl. undulatoplicatus.
Direct correlation, based on the occurrence of Cl. undulatoplicatus, is also possible within the northern Tethyan Realm. This species is widely known from the Caribbean (KAUFFMAN1970), and the Mediterranean province (Spain: LÓPEZ 1992;
Italy: DHONDT & DIENI1990; Bulgaria: TZANKOV 1981; Kopet-Dagh, Turkmenistan: ATABEKIAN
1974). For some reason, Cl. undulatoplicatus did not become established along the southern margins of the Tethys. Documentation from these areas is still highly incomplete, but reports from Egypt by SEIBERTZ (1996) and Tunisia (SALAJ 1987, ROBASZYNSKI& al. 2000) indicate that inoceramids of the Pl. cycloides group may serve as secondary markers for the base of the Santonian there.
Cladoceramids are also present in the East African Province, occurring in KwaZulu-Natal, South Africa (KENNEDY & al. in press) and Madgascar (SORNAY1964, 1969). Based on associ- ated ammonite faunas, the group was regarded here as of middle and even late Santonian age (SORNAY1969). New data from both these regions suggest, however, that the first occurrence of cladoceramids here corresponds to their first occurrence in the Euramerican region (see KENNEDY& al. in press).
Cladoceramids are widely known from the Japanese-East Asian Subprovince of the North Pacific Province where, however, they are appar- ently much younger, and are dated as latest Santonian to early Campanian (MATSUMOTO &
UEDA1962; MATSUMOTO& al. 1982; NODA1983),
or exclusively early Campanian (TOSHIMITSU 1988; TOSHIMITSU & al. 1995, 1998). In inoce- ramid terms the base of the Santonian is defined in Japan by the first occurrence of ‘Inoceramus’
amakusensis, an apparently endemic platyceramid species. Its correlation with the European or East African schemes, i.e., with the base of the undu- latoplicatus Zone, is based on ammonites, specifi- cally on the first appearance of Texanites. But it is now well established that this genus first appeared in the late Coniacian (KAPLAN &
KENNEDY 2000, KENNEDY & KAPLAN 2000), as confirmed here.
Biostratigraphy
The interval studied in the WalMart section on Ten Mile Creek represents two inoceramid zones.
The Coniacian part belongs to the Magadiceramus crenelatus Interval Zone and the Santonian part represents the Cladoceramus undulatoplicatus Taxon Range Zone. The M. crenelatus Zone is dated as late Late Coniacian. The base of the zone is defined by the first occurrence of the index species. The top is defined by the first occurrence of Cl. undulatoplicatus. The zone can be recognised throughout the southern and central parts of the Euramerican biogeographical region, and in the northern Tethyan province. Less clear is the upper stratigraphic boundary of the Cl. undulatoplicatus Zone. In the Euramerican biogeographical region the zone, as currently defined, is regarded as equiv- alent to the Lower Santonian. Single specimens of Cladoceramus have been reported, however, from levels well above the interval with common clado- ceramids, which is taken as marking the top of the zone (SEITZ1965, p. 135). SEITZ(1965) compared these specimens with Cl. japonicus, but their actu- al affinities are currently unresolved. The zone can be recognised throughout the Euramerican bio- geographical region and in the northern Tethyan Realm. Its equivalent in the East African Province is the Cladoceramus hokkaidoensis Zone.
Systematic palaeontology
Genus Magadiceramus HEINZ, 1932 TYPE SPECIES: Inoceramus subquadratus SCHLÜ-
TER, 1887, p. 43.
REMARKS: See discussion in WALASZCZYK &
COBBAN (2006). The genus is known from the Upper Coniacian of the Euramerican region.
Magadiceramus subquadratus (SCHLÜTER, 1887) (Text-figs 7.5, 7.8, 8.7)
1887. Inoceramus subquadratus SCHLÜTER, p. 43.
2006. Magadiceramus subquadratus (SCHLÜTER, 1887);
WALASZCZYK & COBBAN, p. 297, text-figs 12.2- 12.5, 23.4, 33.1, 33.4-33.5, 34.2, 35.4, 38.5 (with syn- onymy).
TYPE: The lectotype is PIB ADKINS1, the original SCHLÜTER specimen figured by ADKINS(1928, pl.
34, fig. 6) from the Austin Chalk of Texas, USA.
MATERIAL: Three specimens: OUM KT10002 from the 10.2 m level, OUM KT10011 from the 5.7 m level, and OUM KT10026 from the 16.7 m level in the WalMart section.
DESCRIPTION: OUM KT10002 is an internal mould of a single LV, with its ventral part missing.
The specimen is moderately large, with L = 97 mm.
The valve is weakly inflated, quite oblique (with δ = 50°). The anterior margin is short, convexly round- ed with the anterior face flattened. The hinge line is long and straight. In the juvenile and early adult stage the valve has an obliquely ovate outline; the subquadrate outline appears later in ontogeny. The posterior auricle is small, triangular. The ornament up to h = 63 mm consists of one generation of con- centric rugae; further ventrally two generations of rugae appear, with the two to three smaller rugae in the interspaces between the main rugae.
OUM KT10011 is a moderately large internal mould of a double-valved specimen, subquadrate in outline, with its postero-dorsal part (including the posterior auricle) missing. The specimen is geniculated. Umboward of the geniculation the disc is weakly inflated. Posterior to the growth axis there is a narrow, step-wise radial sulcus. The ornament consists of concentric rugae typical for Magadiceramus, with superimposed, raised growth lines.
OUM KT10026 is an internal mould of a quite large LV. The juvenile stage (umboward of the genic- ulation) of the valve is oval in outline, resembling a Platyceramus; the postero-dorsal part is missing.
Fig. 7. Inoceramids from the Bruceville Member of the Austin Chalk of the WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. 1-2, 6-7, 9 – Magadiceramus crenelatus (SEITZ, 1970); 1 – OUM KT10021, 11.3 m level; 2 – OUM KT10013, 11 m level; 6 – OUM KT10020, 11 m level; 7 – OUM KT10010, 1 m level; 9 – OUM KT10014, 11 m level. 3-4 – Magadiceramus complicatus (HEINE, 1929); 3 – OUM KT10016, 11 m level; 4 – OUM KT10012, 5.7 m level. 5, 8 – Magadiceramus subquadratus (SCHLÜTER, 1887); 5 – OUM KT10011,
5.7 m level; 8 – OUM KT10002, 10.2 m level. All figures are natural size
1
8 9
7 6
4
2
5
3
REMARKS: The status of the currently recog- nised species of Magadiceramus still needs further clarification. Magadiceramus crenelatus, M. compli- catus and the nominate subspecies are separated on the basis of their ornament, but the mutual dif- ferences are often very slight. Moreover, juveniles or juvenile fragments, which dominate collections, are indistinguishable, so that determining the actu- al record of species is often difficult. Because the stratigraphical appearance of crenelation and of radial ribs appears to be the same in all successions described, a simple morphological (typological) concept is applied here. The consequent extension of morphological variability within species as cur- rently defined indicates that the original subspecies level categories as applied to these taxa by SEITZ
(1970) cannot be maintained.
OCCURRENCE: Magadiceramus subquadratus is known from most of the Euramerican biogeo- graphic region, although apparently absent in its most northerly parts. It ranges through the entire Upper Coniacian.
Magadiceramus complicatus (HEINE, 1929) (Text-figs 7.3, 7.4, 8.6)
1929. Inoceramus subquadratus var. complicata HEINE, p.
38, pl. 2, fig. 7.
2006. Magadiceramus complicatus (HEINE, 1929); WALASZ-
CZYK& COBBAN, p. 303, figs 30.1-3, ?30.4, 30.5, ?31.5 (with synonymy).
TYPE: The lectotype is GPIM B666, the original of HEINE (1929, pl. 2, fig. 7) from the Upper Coniacian of the Wetterschacht of the Grimberg Mine 3, Westphalia, Germany, at a depth 140 m.
MATERIAL: Three specimens: OUM KT10012 from the 5.7 m level; OUM KT10016 and KT10025b from the 11 m level in the WalMart section.
DESCRIPTION: All specimens studied are
incomplete internal moulds of single valves. They have well-preserved ornament. The best preserved are OUM KT10016 and KT10025. Both specimens have a subquadratus type juvenile stage (in OUM KT10025 it becomes transitional to crenelatus type) followed by radial ribbing that produces the complex ornament of the adult stage. The radial ribs are strongest at or near the edges of the con- centric rugae. OUM KT10012 is an antero-ventral fragment of an adult, with well-developed radial ornament.
REMARKS: The present specimens are typical representatives of M. complicatus, comparing well with the type (HEINE1929, pl. 2, fig. 7). The species resembles Inoceramus americanus WALASZCZYK&
COBBAN(2006, p. 263, figs 11.1, 11.7, ?12.9, 12.13- 12.15, 23.2), from which it differs in its sub- quadrate valve outline.
OCCURRENCE: Upper Upper Coniacian of the Euramerican biogeographical region.
Magadiceramus crenelatus (SEITZ, 1970) (Text-figs 7.1, 7.2, 7.6, 7.7, 7.9, 8.3, 8.4) 1929. Inoceramus subquadratus SCHLÜTER; HEINE, p. 34,
pl. 1, figs 1-2.
2006. Magadiceramus crenelatus (SEITZ, 1970); WALASZ-
CZYK& COBBAN, p. 301, text-figs 33.2, 33.6, 33.8;
34.4, 36.1 (with synonymy).
TYPE: The holotype, by original designation, is GWB S 528 Wb, the original of SEITZ(1970, pl. 4, fig. 2) from the Upper Coniacian of mine section 5 of Minister Stein, Westphalia, Germany.
MATERIAL: UWFG ZI/31/081 and OUM KT10010 from the 1 m level; OUM KT10013/18 (part and counterpart), OUM KT10014, OUM KT10017, OUM KT10019, and OUMKT10020 from the 11 m level; OUM KT10021 from the 11.3 m level; UWFG ZI/31/082, from just below the 18.4 m candidate GSSP level.
Fig. 8. Inoceramids from the Bruceville Member of the Austin Chalk of the WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. 1, 5 – Magadiceramus cf. crenistriatus (HEINZ, 1928); 1 – UWFG ZI/31/080, 18.4 m level; 5 – OUM KT10024, 11.3 m level. 2 – Magadiceramus sp.; OUM KT10025a, 11 m level. 3, 4 – Magadiceramus crenelatus (SEITZ, 1970); 3 – UWFG ZI/31/081, most probably from the 1 m level; 4 – UWFG ZI/31/082, 18.4 m level. 6 – Magadiceramus complicatus (HEINE, 1929); OUM KT10025b,
11 m level. 7 – Magadiceramus subquadratus (SCHLÜTER, 1887); OUM KT10026, 16.7 m level. All figures are natural size
1
2 3
4 5
6 7
DESCRIPTION: All specimens are moulds of sin- gle valves. UWFG ZI/31/081, OUM KT 10010 and OUM KT 10020 are quite complete, with well- pre- served details of ornament. All specimens are sub- quadrate in outline with a distinct radial sulcus, developed posteriorly of the growth axis. The sul- cus is wide and shallow. The posterior auricle is narrow, elongated parallel to the hinge line, and separated from the disc with a distinct step, as is well seen in OUM KT10020 and OUM KT10018/13. The other specimens are fragments of adults with crenelated portions preserved.
Crenelation starts at various distances from the umbo. When present, it is usually developed sub- regularly on all of the main rugae. The interspaces bear well-developed, sharp-edged growth lines.
Juveniles and young adults of M. crenelatus are indistinguishable from those of M. complicatus.
OCCURRENCE: This species first appears slight- ly later than M. subquadratus and appears to extend almost to the top of the Coniacian throughout most of the Euramerican biogeographic region, apart from the most northerly parts (see COBBAN& al.
2005; WALASZCZYK& COBBAN2006).
Magadiceramus cf. crenistriatus (HEINZ, 1928) (Text-figs 8.1, 8.5)
Compare:
pars 1909. Inoceramus subquadratus ROEMER; SCHROE-
DER, p. 63, pl. 16, fig. 1.
2006. Magadiceramus crenistriatus (HEINZ, 1928);
WALASZCZYK& COBBAN, p. 302, figs 28.3, 28.7.
MATERIAL: Two specimens: OUM KT10024 from the 11.3 m level and UWFG ZI/31/080, from the 18 m level in the WalMart section.
DESCRIPTION: Both specimens are adult, frag- mentary internal moulds. OUM KT10024 is the antero-ventral part of an individual, and is appar-
ently undeformed, UWFG ZI/31/080 is an adult fragment with the juvenile part missing, and the anterior margin deformed. Both specimens show the typical crenistriatus ornament, composed of rel- atively widely spaced concentric rugae and radial ribs. There are two generations of concentric rugae: one or two smaller second-generation rugae are present between successive larger first-genera- tion rugae, forming a reticulate ornament. As in Magadiceramus complicatus, the radial ribs occur only on the anterior half of the disc, disappearing at the radial sulcus and absent posterior of it.
REMARKS: Magadiceramus crenistriatus differs from the other Magadiceramus species in its orna- ment, which is composed of strong and widely spaced concentric rugae, with smaller rugae in between and with superimposed radial ribs, vari- ously developed, together producing the character- istic reticulate ornament.
OCCURRENCE: Upper Upper Coniacian of the US Western Interior, Texas and Germany.
Genus Cordiceramus HEINZ, 1932
TYPE SPECIES: Inoceramus cordiformis J. DEC.
SOWERBY, 1823, p. 61, pl. 44.
OCCURRENCE: Cordiceramus first appeared in the Late Coniacian (e.g. NODA1986) and ranged into the mid-Campanian (SEITZ 1961, 1967;
WALASZCZYK& al. 2001; WALASZCZYK2004).
Cordiceramus cf. cordiformis (J. De C. SOWERBY, 1823)
(Text-figs 9.5, 9.9) Compare:
1823. Inoceramus cordiformis J. DEC. SOWERBY, p.
61, pl. 44.
Fig. 9. Inoceramids from the Bruceville Member of the Austin Chalk of the WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. 1-3, 6, 10 – Cordiceramus arnoldi (SEITZ, 1961); 1 – OUM KT10044, 22 m level; 2 – OUM KT10041, 21.2 m level; 3 – OUM KT10040, 21.2 m level; 6 – UWFG ZI/31/084, from slightly above the 22 m level; 10 – OUM KT10045, 22 m level;. 4 – Cladoceramus undulatopli- catus (ROEMER, 1852); UWFG ZI/31/083, 18.5 m level. 5, 9 – Cordiceramus cf. cordiformis (J. DEC. SOWERBY, 1823); 5 – UWFG ZI/31/088, 22 m level; 9 – OUM KT10043, 22 m level. 7 – Platyceramus ex gr. mantelli (DEMERCEY) BARROIS, 1879, UWFG ZI/31/085, from slight- ly above the 18 m level. 8 – Inoceramus cf. anomalus (HEINE, 1929), OUM KT10032, from the 18.35 m level. All figures are
natural size
1
5
2 3
4
6
7
9
8 10
1912. Inoceramus cordiformis SOWERBY; WOODS, p.
334, pl. 53, fig. 8; pl. 54, fig. 4.
1961. Inoceramus (Cordiceramus) cordiformis cordi- formis SOW.; SEITZ, p. 114, text-fig. 26a, b.
pars 1961. Inoceramus (Cordiceramus) cordiformis subsp. indet.; SEITZ, p. 120, pl. 8, fig. 5.
TYPE: The holotype is the original of J. DE C.
SOWERBY (1823, pl. 44, re-illustrated by WOODS
1912, pl. 53, fig. 8), from the (inferred) Santonian of Gravesend, England.
MATERIAL: Two specimens: UWFG ZI/31/088 and OUM KT10043 from the 22 m level in the WalMart section.
DESCRIPTION: TMC 38 is a small (h max = 30 mm) internal mould of a right valve. The posterior auricle is not preserved. It has a moderately long anterior margin and distinctly sulcate ventral one.
The disc is moderately oblique (precise measure- ment cannot be made). There is a shallow but dis- tinct radial sulcus posterior to the growth axis, which becomes progressively stronger ventrally.
The ornament consists of weak, round-edged rugae and distinct, raised growth lines.
OUM KT10034 is a juvenile fragment of a sin- gle left valve, with portions of the shell preserved.
The umbonal part and posterior auricle are not seen. The length of the exposed part is 38 mm. The disc is subtriangular, slightly oblique. Posteriorly of the growth axis there is a weak radial sulcus. The disc surface is ornamented with concentric, round- edged rugae, which become stronger when traced ventrally. The rugae appear to have been weak or absent in the umbonal region.
REMARKS: Valve outline, presence of the posteri- or disc sulcus, and the same type of ornament, show these specimens to be Cordiceramus cordiformis (J.
DEC. SOWERBY, 1823) in the narrow sense of the nominate subspecies C. cordiformis cordiformis of SEITZ(1961). The specimens resemble the smallest individual illustrated by WOODS(1912, pl. 54, fig.
4), from Micheldever, Hampshire, England. Also referred here is the original of SEITZ (1961, pl. 8, fig. 5), unassigned to subspecies by him, from the basal Middle Santonian.
OCCURRENCE: The present specimens are from the 22 m level in the Lower Santonian. Cordi-
ceramus cordiformis, in the narrow sense of SEITZ, is known from the Lower and Middle Santonian of the Euramerican biogeographic region. SEITZ
(1961, 1967) recorded it from the Lower, Middle and lower Upper Santonian of Germany.
Cordiceramus ex gr. cordiformis (J. DEC. SOWERBY, 1823)
(not illustrated)
MATERIAL: UWFG ZI/31/089 from the 21.2 m level in the WalMart section.
DESCRIPTION AND REMARKS: UWFG ZI/31/089 is an internal mould of a single incom- plete left valve. The beak part is missing. The valve is subquadrate, with a long anterior margin, pass- ing sharply into a long, broadly convex ventral mar- gin. The anterior face is steep and moderately high. The posterior auricle is small, triangular and well separated from the disc. The ornament is composed of round-edged, subregularly spaced concentric rugae.
The specimen differs from C. cordiformis in the narrow sense, in that it lacks the disc sulcus, more closely resembling either subspecies gravis or stimmbergensis of SEITZ.
OCCURRENCE: Lower Santonian, 21.2 m level in the WalMart section.
Cordiceramus arnoldi (SEITZ, 1961) (Text-figs 9.1-9.3, 9.6, 9.10)
1953. Inoceramus cycloides WEGNER; ØDUM, p. 14, pl. 2, fig. 6.
1961. Inoceramus (Cordiceramus) bueltenensis arnoldi SEITZ, p. 147, pl. 11, figs 1-4, 7; pl. 13, fig. 2.
2006. Cordiceramus arnoldi (SEITZ, 1961); WALASZCZYK
& COBBAN, figs 35.1-35.3, 36.2, 37.5 (with syn- onymy).
TYPE: The holotype, by original designation, is NLBH Ko 456, the original of SEITZ(1961, pl. 11, fig. 2) from the Lower Santonian of shaft 8 of the Hugo Mine, Westphalia, Germany.
MATERIAL: OUM KT10040 and OUM KT10041 from the 21.2 m level; OUMKT10044, OUM
KT10045 and UWFG ZI/31/084, from the 22 m level of the WalMart section.
DESCRIPTION: All specimens are internal moulds of single left valves of the juvenile inflated stage. OUM KT10040 and KT10041 are small specimens, with h max = 32 and 36 mm, respec- tively. They are preserved to the first geniculation, with a fragment of a part of the valves between the first and second geniculation. Their postero-dorsal portions are not preserved. Up to the first genicu- lation the valve is ornamented with fine concentric rugae, with interspaces apparently increasing ven- trally. Ventrally of the geniculation, the ornament changes to distinctly stronger rugae, with larger interspaces. Fragments OUM KT10044 and UWFG ZI/31/084 are larger (h max = 50 and 44 mm, respectively), and their ornament is com- posed of markedly stronger rugae, with interspaces reaching up to 7 mm (versus 3 mm in OUM KT 10040 and KT10041).
REMARKS: The problems surrounding Cordi- ceramus arnoldi have recently been discussed by WALASZCZYK& COBBAN(2006) and their interpre- tation is followed here.
OCCURRENCE: Cordiceramus arnoldi first appears very close to the base of the Santonian. In the US Western Interior the species is known from the Lower Santonian of the Pueblo section (start- ing at USGS Denver Mesozoic Locality D3490 and ranging up to D3495: WALASZCZYK & COBBAN
2006, text-fig. 2), and from the Lower Santonian (Clioscaphites saxitonianus ammonite Zone) of the Cody Shale, Carbon County, Montana (see WALASZCZYK & COBBAN 2006). It is also well known from the Lower and Middle Santonian of Europe (SEITZ1961).
Genus Platyceramus HEINZ, 1932
TYPE SPECIES: Inoceramus mantelli (DE
MERCY) BARROIS, 1879, p. 454, pl. 4, figs 1, 2.
OCCURRENCE: The earliest representatives of Platyceramus are known from the mid-Lower Coniacian; the genus probably ranges to the mid- Maastrichtian, and has a cosmopolitan distribu- tion.
Platyceramus ex gr. mantelli (DEMERCEY) BARROIS, 1879
(Text-fig. 9.7) Compare:
1962. Inoceramus (Platyceramus) mantelli MERCEY (BARROIS); SEITZ, pp. 355-369; pls 10-13.
1985. Inoceramus mantelli mantelli MERCEY, 1872;
SZÁSZ, p. 171, pl. 33, figs 1-2; pl. 34, fig. 1.
1985. Inoceramus mantelli beyenburgi SEITZ; SZÁSZ, p.
171, pl. 33, fig. 3; pl. 34, fig. 2; pl. 40, fig. 1.
TYPE: The lectotype, by subsequent designation of SEITZ(1962, p. 356), is the original of BARROIS (1879, pl. 4, fig. 1; photographically illustrated by SEITZ1962, pl. 10, fig. 1a) from the Coniacian of Lezennes, near Lille, northern France.
MATERIAL: UWFG ZI/31/085, from just below the candidate GSSP level at 18 m in the WalMart section.
DESCRIPTION AND REMARKS: UWFG ZI/31/085 is an internal mould of a single left valve.
The specimen is rather poorly preserved with its beak and posterior parts missing. The general out- line and ornament, composed of subregular, low concentric rugae, with raised growth lines allow to refer it to the mantelli group.
OCCURRENCE: This specimen was found just below the candidate GSSP level in the WalMart section. Elsewhere, P. mantelli appears near the base of the Middle Coniacian and ranges almost the top of the stage. The species is known from the Euramerican biogeographical region and from the North Pacific Province.
Platyceramus cycloides (WEGNER, 1905) (Text-fig. 10.1)
1905. Inoceramus cycloides nov.sp., WEGNER, p. 162, pl.
7, fig. 3 and text-fig. 6.
2006. Platyceramus cycloides (WEGNER, 1905); WALASZ-
CZYK& COBBAN, p. 294, text-figs 32.1, 34.1.
TYPE: The lectotype, indicated by BÖHM (1915) and formally designated by SEITZ(1961, p. 55), is the original of WEGNER(1905, p. 163, text-fig. 6 and pl. 7, fig. 3) from the Santonian of the Blumenthal
Fig. 10. Inoceramids from the Bruceville Member of the Austin Chalk of the WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. 1 – Platyceramus cycloides (WEGNER, 1905), UWFG ZI/31/086, 22 m level. 2 – Cladoceramus undulatoplicatus (ROEMER,
1852), UWFG ZI/31/090, 18.5 m level. Both figures are natural size
1
2
mineshaft (Schacht V), in the Münsterland Basin, northern Germany.
MATERIAL: UWFG ZI/31/086, from the 22 m level in the WalMart section.
DESCRIPTION: The specimen is an internal mould of a right valve, incomplete in the ventral and postero-ventral parts. The valve is sub- quadrate, moderately oblique and almost flat;
these features are to some extent due to secondary lateral compression. The beak is small, pointed, projecting slightly above the hinge line, which is straight and long. The anterior margin is broadly convex, relatively long (61% of the respective axial length). The ornament is composed of closely and regularly spaced concentric rugae, with l/h ratio approximating 95 %. Only the rugate part of the specimen is preserved. However, as indicated at the posterior part of the valve, the regularly rugate juvenile part of the valve was followed by the smooth to irregularly rugate adult stage.
REMARKS: The specimen is referred to Platyceramus cycloides on the basis of its ornament, the l/h ratio and the valve outline.
OCCURRENCE: The specimen is from the 22 m level, 3.6 m above the base of the Santonian. The species is known to appear first close to the base of the Santonian in Europe (SEITZ1961); it apparently ranges up to the top of the stage, and possibly higher.
Genus Cladoceramus SEITZ, 1961
TYPE SPECIES: Inoceramus undulatoplicatus var.
michaeli HEINZ, 1928 (=Inoceramus digitatus SCHLÜTER, 1877, non Inoceramus digitatus J.DEC.
SOWERBY, 1829).
REMARKS: See discussion in WALASZCZYK &
COBBAN(2006).
Cladoceramus undulatoplicatus (ROEMER, 1852) (Text-figs 1, 9.4, 10.2)
1849. Inoceramus undulato-plicatus ROEMER, p. 402.
1852. Inoceramus undulato-plicatus ROEMER, p. 59, pl. 7, fig. 1.
2006. Cladoceramus undulatoplicatus (ROEMER);
WALASZCZYK& COBBAN, p. 308, figs 39.1-3, 40.4 (with synonymy).
TYPE: The lectotype is PIB ROEMER Collection 03, the original of ROEMER(1852, pl. 7, fig. 1; see Text-fig. 1 herein) from the Lower Santonian undulatoplicatus Zone of the Austin Chalk, on the banks of the Guadalupe River, below New Braunfels, Texas, USA.
MATERIAL: UWFG ZI/31/083, ZI/31/090, and a numerous fragmentary specimens from the 18.4 to 19 m level of the WalMart section.
DESCRIPTION AND DISCUSSION: All of the three morphotypes within the species recognised by SEITZ(1961) are present in the Texas material:
(1) the slender forms, similar to the lectotype; (2) subrectangular to subquadrate forms, referred by SEITZ to Cl. undulatoplicatus ssp. indet., and (3) michaeli morphotypes, with asymmetrical radial ornament pattern.
The species is referred herein to the genus Cladoceramus following recent discussion by WALASZCZYK(in KENNEDY& al. in press).
OCCURRENCE: In the sections studied, the species first occurs at the 18.4 m candidate basal Santonian GSSP level in the WalMart section, where it is quite common. It becomes less common in the higher parts of the WalMart succession. A sin- gle specimen, referred herein to as Cl. cf. undulato- plicatus comes from slightly above the 19 m level.
Elsewhere in the sections studied here, there is a second abundance peak in the Bruceville Member:
between 12.5 and 14 m in the Nazarene 2 section, and between 4.6 and 8 m in the Arbor Park section (Text-fig. 4). The species is known from the Lower Santonian throughout the Euramerican biogeo- graphic region, and from the East African Province.
AMMONITES (W.J. KENNEDY)
Order Ammonoidea VONZITTEL, 1884 Suborder Ammonitina HYATT, 1889 Superfamily Acanthoceratoidea DEGROSSOUVRE,
1894
Family Collignoniceratidae WRIGHT& WRIGHT, 1951
Subfamily Texanitinae COLLIGNON, 1948 Genus and Subgenus Protexanites MATSUMOTO,
1955
TYPE SPECIES: Ammonites bourgeoisianus
D’ ORBIGNY, 1850, p. 212, by the original desig- nation of MATSUMOTO(1955, p. 38).
Protexanites (Protexanites) planatus (LASSWITZ, 1904)
(Text-figs 11, 12)
1904. Schloenbachia quattornodosum var. planata LASSWITZ, p. 32, pl. 7, fig. 4.
1963. Protexanites planatus (LASSWITZ, 1904); YOUNG, p.
76, pl. 26, figs 3, 4; pl. 35, fig. 4; pl. 36, figs 1, 2; pl.
37, figs 2-4; text-figs 20a, 25 m, 29c (with addition- al synonymy).
1991. Protexanites planatus (LASSWITZ, 1904); LARSON&
al., fig. 10.3.
1991. Paratexanites sellardsi YOUNG, 1963; LARSON& al., fig. 10.4.
MATERIAL: Two specimens in the P. A. LARSON
collection, Dallas, from the 10.3 m level in the Atco Formation in the Nazarene 1 section (Text-fig. 4).
TYPE: The holotype, by monotypy, is the original of LASSWITZ (1904, pl. 7, fig. 1), reillustrated by
Fig. 11. Protexanites (Protexanites) planatus (LASSWITZ, 1904). A specimen in the P.A.LARSONcollection, Dallas, from the 10.3 m level in the Upper Coniacian Atco Member of the Austin Chalk of the Nazarene 1 section on Ten Mile Creek, Desoto, Dallas County,
Texas. The figure is natural size
ADKINS(1928, pl. 34, fig. 3), from the Capitol exca- vations in Austin, Texas. The holotype appears to have been destroyed in the Second World War, and could not be found in the collections of the Henryk Teisseyre Museum in Wroc∏aw in 1993 (Dr. J.
GORYCZYCA-SKALAin a letter to Dr. H.C. KLINGER
dated 2.04.1993).
DESCRIPTION: There are two specimens from the Nazarene 1 section on Ten Mile Creek. The
smaller (Text-fig. 11) is the original of LARSON& al.
1991, fig. 10.3, a rather battered individual with a maximum preserved diameter of 155 mm. Coiling is very evolute, with the umbilical wall crenulated to accommodate the submarginal tubercle of the pre- ceding whorl. The broad umbilicus comprises 42%
of the diameter and is shallow, with a rounded, undercut wall and broadly rounded umbilical shoul- der. The original whorl section cannot be recon- structed, because of extensive post-mortem crushing.
Fig. 12. Protexanites (Protexanites) planatus (LASSWITZ, 1904). A specimen in the P.A. LARSONcollection, Dallas, from the 10.3 m level in the Upper Coniacian Atco Member of the Austin Chalk of the Nazarene 1 section on Ten Mile Creek, Desoto, Dallas
County, Texas. The figures are natural size
There are fifteen coarse, very distant, straight pror- siradiate ribs on the outer whorl. They arise at the umbilical seam and strengthen across the umbilical wall and shoulder, where they are feebly concave.
The ribs coarsen markedly across the flanks. There is doubled umbilical tuberculation with a weak umbilical and stronger inner lateral node. The mar- ginal tubercle is massive and conical-clavate, the external tubercle weaker and markedly clavate. A pronounced groove separates the external clavi from the strong, undulose siphonal keel, such that the external clavi appear to be linked by a weak ridge. The second specimen (Text-fig. 12) is much better preserved, and is the original of LARSON&
al. (1991, fig. 10.4), where it was referred to as Paratexanites sellardsi YOUNG, 1963. The specimen is a crushed composite mould with a maximum pre- served diameter of 170 mm. The coiling is very evo- lute, with the umbilical wall notched to accommo- date the marginal tubercle of the previous whorl.
The wide umbilicus comprises 42% of the diame- ter, the umbilical wall is convex, the umbilical shoulder inclined outward. The whorl section is very compressed, as a result of post-mortem crush- ing. At a diameter of 96 mm there are 15-16 coarse, distant primary ribs per whorl. They arise at the umbilical seam and strengthen across the umbilical wall and shoulder, where they are feebly concave.
They are straight and prorsirardiate and strengthen progressively across the flanks. There is a coarse umbilicolateral bulla, displaced out from the umbil- ical shoulder, and a stronger conical-clavate mar- ginal tubercle. There are 20-21 coarse, distant pri- mary ribs on the outer whorl. The umbilicolateral tubercles weaken progressively, and show a slight outward migration. An incipient umbilical bulla appears. The marginal tubercles weaken progres- sively; one is incipiently doubled. There are very elongate external clavi, separated by a groove from the coarse, undulose siphonal keel. The penulti- mate rib bears what appears to be a feeble lateral tubercle.
DISCUSSION: The smaller specimen differs in no significant respects from the holotype, having the same rib density and the conspicuous doubling of umbilical and umbilicolateral tubercles. The larger specimen has inner whorls as coarsely ribbed as in the holotype, but without showing the conspicuous doubling of the umbilical/umbilicolateral tuber- cles. The outer whorls, with a higher rib density,
match those of the well-preserved specimen figu- red by YOUNG(1963, pl. 37, fig. 2).
LARSON& al. (1991) referred this specimen to Paratexanites sellardsi. YOUNGstates this species to have a doubled marginal tubercle. This is not developed in the present specimen except for a sin- gle rib, and the specimen has more characteristics of Protexanites than of Paratexanites.
OCCURRENCE: Upper Coniacian, Prionocyclo- ceras gabrielense Zone of YOUNG (1963) in Williamson County, Texas. The present specimens are from the 10.3 m level in the Atco Member of the Nazarene 1 section.
Genus and subgenus Texanites SPATH, 1932 TYPE SPECIES: Ammonites texanus ROEMER (1852, p. 31, pl. 3, fig. 1), by the original designa- tion of (SPATH1932, p. 379).
Texanites (Texanites) gallicus COLLIGNON, 1948 (Text-figs 13, 14)
1894. Mortoniceras texanum F. ROEMER sp.; DE
GROSSOUVRE, p. 80, pl. 16, figs 2, 4; pl. 17, fig. 1.
1987. Texanites (Texanites) gallicus COLLIGNON, 1948;
KENNEDY, p. 770, pl. 80, figs 4-7; pl. 81, figs 1-6 (with synonymy).
1994. Texanites (Texanites) gallicus COLLIGNON; WIED-
MANNin GISCHLERet al., p. 238, pl. 44, figs 1, 3, pl.
35, figs 3-6; text-fig. 16e.
1995. Texanites (Texanites) gallicus COLLIGNON, 1948;
KENNEDY, p. 420, pl. 22, fig. 11; text -fig. 25.
TYPE: Lectotype, by the subsequent designation of COLLIGNON(1948, p. 42 (99)) is the original of DE GROSSOUVRE (1894, pl. 17, fig. 1), from the
‘Marnes Bleues ∫ petits fossiles situées au bas du Chemin de Sougraine aux Croûtets (Aude) sous la couche ∫ Lima marticensis’.
MATERIAL: Three specimens, OUM KT9368, from the 16 m level, OUM KT9366, from the 21.2 m level, and OUM KT9367, from the 22.1 m level in the Bruceville Member in the WalMart section.
DESCRIPTION: Of the three specimens, the best preserved is OUM KT9366 (Text-fig. 13), a very
crushed composite mould with a maximum pre- served diameter of 140 mm. Coiling is very evolute, the umbilicus broad and shallow, comprising 40%
of the diameter, the umbilical wall convex, the umbilical shoulder broadly rounded. There are 23 strong, prorsiradiate ribs on the outer whorl, all primaries. There are well-developed bullae perched on the umbilical shoulder, rounded to fee- bly clavate tubercles at the junction of the middle and inner third of the flank, weaker, clavate sub- marginal tubercles, stronger, clavate marginal tubercles and external clavi, separated by a groove from the siphonal keel. OUM KT 9367 (Text-fig.
14) is a crushed juvenile with an estimated original diameter of 76-80 mm. OUM KT 9368 (not fig- ured) comprises a nucleus 60 mm in diameter and
a fragment of the succeeding whorl with a maxi- mum preserved whorl height of 48 mm.
DISCUSSION: These rather poor specimens are referred to the widespread Texanites (Texanites) gallicus, previously recorded from Texas by YOUNG
(1963). They differ most obviously from the larger specimen from Ten Mile Creek described below as T. (T.) vanhoepeni in their lower expansion rate.
OCCURRENCE: The present specimens are from the Bruceville Member in the WalMart section;
OUM KT9368 is from the 16 m level, and is Upper Coniacian; OUM KT9366 is from the 21.2 m level, and OUM KT9367 is from the 22.1 m level and both are Lower Santonian. The species is also
Fig. 13. Texanites (Texanites) gallicus COLLIGNON, 1948. OUM KT 9366, from the 21.2 m level in the Lower Santonian part of the Bruceville Member of the WalMart section on Ten Mile Creek, Lancaster, Dallas County, Texas. The figures are natural size
known from southern France, Austria, Spain, Hokkaido, Japan, Madagascar, Zululand (South Africa) and, doubtfully, Angola. It has traditional- ly been regarded as Lower Santonian in these areas.
Texanites (Texanites) vanhoepeni KLINGER&
KENNEDY, 1980 (Text-fig. 15)
1980. Texanites (Texanites) vanhoepeni KLINGER &
KENNEDY, p. 144, figs 109-117.
1990. Texanites sp. cf. texanus (ROEMER); LARSON& al., fig. 10.6.
MATERIAL: A specimen in the P. A. LARSON Collection, Dallas, from the 10.3 m level in the Atco Formation in the Nazarene 1 section (Text- fig. 4).
DESCRIPTION: The specimen, previously illus- trated by LARSON& al. (1990, fig. 10.6) is a very crushed section of a 120° sector of body chamber with a maximum preserved whorl height of 75 mm. Parts of nine ribs are preserved on the frag- ment. They are coarse, low, broad and straight,
with sharp umbilical bullae, strong, rounded lat- eral, strong clavate submarginal amd marginal tubercles and weaker external clavi. There is a strong siphonal keel. The umbilical, lateral, sub- marginal and marginal rows are approximately equidistant.
DISCUSSION: The fragment appears to have a relatively higher whorl and a smaller umbilicus than Texanites (Texanites) gallicus. It closely resem- bles the holotype (KLINGER& KENNEDY, 1980, figs 109-111).
OCCURRENCE: The types are from the Upper Coniacian to Lower Santonian of Zululand. The present specimen is from the Upper Coniacian 10.3 m level in the Atco Member of the Nazarene 1 section. An external mould comparable to this specimen was observed at the 1.7 m level in the Bruceville Member in the WalMart section, and is also Upper Coniacian.
Genus Menabites COLLIGNON, 1948 TYPE SPECIES: Menabites menabensis COLLI-
GNON, 1948, p. 7 (64), pl. 17, figs 3, 4; pl. 18, fig.
1, by original designation by COLLIGNON, 1948, p. 64 (19).
Subgenus Australiella COLLIGNON, 1948 TYPE SPECIES: Mortoniceras australe BESAIRIE, 1930, pl. 64, fig. 2, by original designation by COLLIGNON, 1948, p. 64 (19).
Menabites (Australiella) austinensis YOUNG, 1963 (Text-fig. 16)
1963. Australiella austinensis YOUNG, p. 115, pl. 64, figs 3, 4; pl. 65, fig. 6; pl. 67, figs 4-6; text-fig. 28e (with synonymy).
TYPE: The holotype is TMM WSA-65, the origi- nal of YOUNG (1963, pl. 65, fig. 6; pl. 67, fig. 6), from the Austin Chalk one and a half miles (2.4 km) southeast of Austin, Texas. According to YOUNG(1963, p. 116) it is probably from the Lower Santonian, but could be Lower Campanian.
Fig. 14. Texanites (Texanites) gallicus COLLIGNON, 1948. OUM KT 9367, from the 22.1 m level in the Lower Santonian part of the Bruceville Member of the WalMart section on Ten Mile Creek,
Lancaster, Dallas County, Texas. The figures are natural size
MATERIAL: OUM KT10046, from the 22.5 m level in the Bruceville Member in the WalMart sec- tion.
DESCRIPTION: The specimen is a very crushed composite mould with an estimated maximum preserved diameter of 100 mm. The umbilicus comprises an estimated 35% of the diameter.
There are an estimated 7-9 well-developed umbil-
ical bullae on the outer half whorl. These give rise to low, broad, straight, prorsiradiate ribs, while additional ribs intercalate on the flank. All ribs bear strong, conical to feebly clavate inner ventro- lateral tubercles. There are numerous small, very elongate outer ventrolateral clavi that are two to three times as numerous as the inner ventrolater- al tubercles. There is a coarse, feebly undulose siphonal keel.
Fig. 15. Texanites (Texanites) vanhoepeni KLINGER& KENNEDY, 1980. A specimen in the P. A. LARSONCollection, Dallas, from the 10.3 m level in the Upper Coniacian Atco Formation in the Nazarene 1 section on Ten Mile Creek, Desoto, Dallas County, Texas.
the figures are slightly reduced
DISCUSSION: Although crushed, this specimen can be referred to M. (A.) austinensis, a species characterised by the presence of three rows of tubercles throughout the known ontogeny. This is a problematic form; all other well-dated Menabites (Australiella) are Campanian.
OCCURRENCE: Lower Santonian, 22.5 m level in the Bruceville Member in the WalMart section;
Santonian or Lower Campanian of the Austin area, Texas.
CALCAREOUS NANNOFOSSILS (J.A. LEES) Introduction
Although calcareous nannofossil data were presented at the Cretaceous Stage Boundaries Symposium held in Copenhagen in 1983 (PERCH- NIELSEN 1983), no particular nannofossil events
were discussed or published in relation to the boundary in the proceedings of the meeting (Bulletin of the Geological Society of Denmark 33 (1-2)). It was further accepted, at the 1995 Brussels Symposium, that there were no pub- lished nannofossil events that approximated the recommended inoceramid boundary event, the FO of Cladoceramus undulatoplicatus (LAMOLDA
& HANCOCK1996), but it was suggested that this event lay in SISSINGH’S (1977) Nannofossil Zone CC15 in the Ten Mile Creek section (J.A.
BURNETT, written communication, based on pre- liminary data, in LAMOLDA & HANCOCK 1996), that is, above the first occurrence of Lithastrinus grillii (i.e., equivalent to Nannofossil Subzone UC11a of BURNETT 1998). However, analysis of additional samples from Ten Mile Creek leads to a revision of this position. The boundary has now been determined to lie within the younger Nannofossil Subzone UC11c (which is almost equivalent to Nannofossil Zone CC16 of SISSINGH
1977).
These data are compared to nannofossil results from the other two candidate sections, Olazagutía (Navarra, northern Spain: LAMOLDA & al., 1999;
MELINTE& LAMOLDA2002; HOWE& al. 2007) and Seaford Head (Sussex, southern England:
HAMPTON& al. 2007; HOWE& al. 2007) to see how well the identified nannofossil events correlate over distance.
Methods and material
The samples comprise soft, bioturbated, pre- dominantly olive-grey marls. Smear-slides were made following the technique described in BOWN
& YOUNG (1998). Semi-quantitative estimates, based on three traverses of each slide, were made using a Zeiss Axio Imager A1 light microscope at a magnification of 1250x. The calcareous nannofos- sil biostratigraphy of twenty-six samples, taken from variable intervals in the composite section from the four outcrops studied (Nazarene 1, WalMart, Nazarene 2, Arbor Park: Text-fig. 4), was determined, using the ‘UC’ biozonation of BURNETT(1998). The defunct ‘CC’ biozonation of SISSINGH (1977), as modified by PERCH-NIELSEN (1985), was also applied to the data. All sample material and slides are stored in the Micro- palaeontology Unit, Department of Earth Sciences, University College London.
Fig. 16. Menabites (Australiella) austinensis YOUNG, 1963. OUM KT 10046, from the 22.5 m level in the Lower Santonian part of the Bruceville Member of the of the Austin Chalk of the WalMart section on Ten Mile Creek, Lancaster, Dallas County,
Texas. The figure is natural size
